Embodiments of the invention relate generally to induction furnaces for heating a workpiece in an inert atmosphere or vacuum and, more particularly, to a system for providing cooling to a workpiece in a uniform fashion, so as to reduce thermal stress in the workpiece.
Conventional induction furnaces include an induction heating system and a chamber that contains a susceptor that is susceptible to induction heating, with the chamber enclosing an inert atmosphere or vacuum therein. An electromagnetic coil sits outside the susceptor and receives high frequency alternating current from a power supply. The resulting alternating electromagnetic field heats the susceptor rapidly. The workpiece to be heated is placed in proximity to and generally within the susceptor so that when the susceptor is inductively heated by the induction heating system, the heat is transferred to the workpiece through radiation and/or conduction and convection. After a desired heating and processing of the workpiece is completed, the workpiece is then subsequently cooled in order to complete the heating/cooling cycle.
With respect to the overall time required to perform the heating/cooling cycle, it is recognized that the cooling time is a very key factor in the overall cycle time. Thus, it is desirable to be able to reduce the cooling time that is necessary for cooling the workpiece to a desired temperature. As a means for decreasing the cooling time, some prior art systems introduce an inert cooling gas that helps to increase the rate of cooling of the workpiece. While the inert cooling gas may be effective at increasing cooling rates compared to static vacuum cooling, the use of such cooling gas may undesirably lead to a build-up of thermal stress in the workpiece. That is, as typical vacuum furnaces have only a single port for gas entry, the side of the workpiece placed next to the location the gas enters the hot zone will cool very quickly compared to the side that is shielded from the gas. This thermal mismatch that is created in the workpiece is a source of the thermal stress, with the thermal stress being greatly increased as the temperature difference across the workpiece grows. This stress in the workpiece can lead to premature failure or changes in geometry due to warpage.
It would therefore be desirable to have an induction furnace that provides for a decrease in the cooling time of the workpiece, while providing uniform gas cooling of the workpiece so as to improve workpiece temperature uniformity during the cooling process and thereby decrease thermal stress in the workpiece.